The udcf driver provides support for the Gude ADS
Expert mouseCLOCK USB and the Expert mouseCLOCK USB II, receivers for the
German DCF77. While receivers for the British MSF time signal station are also
being made, udcf lacks support for them.

udcf implements a timedelta sensor and the delta
(in nanoseconds) between the received time information and the local time can
be accessed through the
sysctl(8) interface. The clock
type is indicated in the sensor description:

DCF77

German DCF77 time signal station (77.5 kHz longwave
transmitter located in Mainflingen near Frankfurt).

The quality of the timedelta is reported as the sensor status:

UNKNOWN

No valid time information has been received yet.

OK

The time information is valid and the timedelta is safe to
use for applications like
ntpd(8).

WARN

The time information is still valid, but no new time
information has been decoded for at least 5 minutes due to a reception or
parity error. The timedelta should be used with care.

CRITICAL

No valid time information has been received for more than
15 minutes since the sensor state degraded from OK to WARN. This is an
indication that hardware should be checked to see if it is still
functional. The timedelta will eventually degrade to a lie as all computer
internal clocks have a drift.

DCF77 uses a 77.5 kHz long wave radio signal transmitted from near Frankfurt,
Germany. Up to about 900 km, the radio signal can travel directly to the
receiver, providing a linearly increasing time offset based on distance. Due
to the curvature of the Earth, beyond this distance the signal must bounce off
the lower ionosphere (residing at approximately 70 km elevation during the
day, and 90 km at night), thus causing a non-linearly increasing time offset
which can only be roughly calculated using trigonometry. Since the distance
and transmission geometry is not known, the clock receivers and
udcf driver currently make no effort to calculate
this offset. We simply assume that the offset is small.

In Germany, the train system uses DCF77 clocks. As the distance from Frankfurt
increases, trains can be expected to run later.